Apparatus for collecting light and its method of manufacture

ABSTRACT

A portable scatterometer and/or an angular radiated light measurement instrument that uses a measurement head which includes a double tapered fiber optic bundle with a concave front face to simultaneously collect partial or full hemispherically scattered light reflected from a point on a surface illuminated by a depolarized, telescopically focused, laser diode source, the light rays being received by each fiber normal to its face. The image of the collected light beams is minifled and coupled by the fiber optic bundle into an anti-blooming CID camera with an x-y scanning area array which converts the light beams to electrical signals. In a unique real time, 486-computer-controlled, data acquisition and reconstruction process, a frame grabber and a unique algorithm are used to collect over 200,000 points of light, reconstruct the data into a 2D or 3D scatter profile, and display the results, all within one second. Multiple embodiments of the tapered fiber optic bundle collector include a full hemispherical version with the laser source directed through the boule, a unitary boule fused from two tapered halves, a linear in-plane tapered bundle and a partial taper with imaging lens mode. Alignment mechanisms provide z-translation, azimuth, and focus adjustments. The measurement unit is in a compact, rugged aluminum housing which is adapted to be secured to a production line machine for a variety of services, such as, a quality control inspection device.

This application is a division of application Ser. No. 08/230,546 filedApr. 20, 1994 now U.S. Pat. No. 5,475,617 issued Dec. 12, 1995 which isa division of original application Ser. No. 07/983,470 filed Nov. 30,1992 now U.S. Pat. No. 5,313,542 issued May 17, 1994.

BACKGROUND OF THE INVENTION

1. Fields of the Invention

The present invention relates generally to an apparatus and method ofmeasuring hemispherical light scattered or emitted from a source, and,more particularly, to a portable scatterometer which uses a doubletapered fiber optic bundle with a concave spherical face, a CID camera,and a frame grabber to hemispherically collect scattered light reflectedfrom a laser illuminated sample and a unique algorithm to rapidlyreconstruct the scatter profile on a computer screen.

2. Discussion of Background and Prior Art

a. Scatterometers

Scatter from optical components reduces signal power, limits resolution,produces noise and has appeared as an unexpected problem in more thanone optical design. Stover, "Optical Scatter: Careful Measurement OfOptical Scatter Provides A Keen Diagnostic", Lasers & Optronics, August1988.

Optical designers and manufacturers require a precise and fasthemispherical light scatter measurement tool because many opticalsurfaces interact with light in unpredictable ways.

A scatterometer is a widely used and extremely valuable tool for opticaldesigners, measuring scattered light from test objects in order todetermine the quality and characteristics of surfaces down to theangstrom level.

In 1987 Breault Research Organization, Inc. ("BRO") introduced amulti-wavelength, surface-scanning, fully automated scatterometer("FASCAT 360") system for use in research and development markets. Thesystem could accommodate up to seven lasers and obtained fullhemispherical measurements of the light reflected from or transmittedthrough a sample, but, only by rotating a photosensitive detector 360degrees about the sample holder while allowing for three-axis (X,Y andZ) rotation and translation of the sample itself (all automated). Theinstrument is a Bi-Directional Reflectometer ("BDR") capable ofmeasuring BiDirectional Reflectance Distribution Functions ("BRDF") andBiDirectional Transmittance Functions ("BTDF") both in-plane andout-of-plane. The system printed 2-D and 3-D plots of the data in realtime and provided unparalleled versatility and dependability.

Until recently, this now conventional technology has limited the rangeof applications of scatterometers because of its large size (a 4'×8'steel top table completely encased in a Center For Radiological Devicesand Health Class 1 housing, called the "truck", with sample accessthrough safety interlock doors and a 386SX computer/laser printersystem), its high cost ($400,000-$600,000) and its long acquisition time(10 minutes to 3 hours to measure, calculate and print a completeanalysis of a sample).

To overcome this size and cost disadvantage, in 1988 Toomay, Mathis &Associates, Inc. ("TMA") introduced a single laser, table mounted,Complete Angle Scatter Instrument ("CASI"™) Class 3 scatterometer, butat great sacrifice to versatility and dependability. While thisinstrument provided three axis rotation and translation (one of whichwas automated) of the sample and 360 degree "sweep" (a differenttechnique than 360 degree rotation about the sample used by FASCAT) ofthe detector, it could only provide 2-D plots of BSDF and still wasrelatively expensive ($97,000-$166,000).

Even more recently, still lower cost, hand-held, battery-powered,microprocessor-controlled scatterometers have been introduced usinghand-held or bench-mounted measurement heads with up to 8 individual,non-movable detectors spaced about the sample which restrictedversatility even more and which further sacrificed reliability andvolume of data. While performing some useful function, these smallerunits are restricted in that they assume the surface to be measured ishomogeneous and that there is no interaction of light which preventshomogeneity. Thus, they are restricted in the number of measurementsthey can make.

The above attempts to design smaller instruments led to instruments thathad much less capability than full scale versions, and, as a result,important information was lost. Applicant has found a solution to thisproblem which minimizes the sacrifices made in capability to achieve thesmall size and yet can measure scattered light on un-nice, non-behaved,non-homogeneous surfaces as did full scale versions, and, not only,without sacrificing versatility and capability, but rather, increasingit. Moreover, none of the prior scatterometers are capable ofspherically simultaneously measuring the angular gradiation of the lightscattered from a source because none of the light collection systems inany of the prior scatterometers were capable of spherically,simultaneously collecting the reflected light scattered from a spot on asurface of the sample. Applicant's unique collection system has solvedthis problem.

b. Fiber Optic Bundles

Fiber optic bundles have been known for many years. See, U.S. Pat. Nos.2,354,591 and 3,033,071 and Siegmund, "Fiber Optic Tapers In ElectronicImaging", Schott Fiber Optics.

A modern fiber optic bundle comprises millions of individual fibers ofglass which are first made by pouring pure raw glass of high index ofrefraction into a tube of lower index of refraction cladding glass, andwhich are then precisely aligned and fused together to form a solidfiber glass bundle ("boule"). Each fiber sees and carries one smallportion of the image by the well known process of internally reflectinglight rays emanating from the image. Through this process highresolution images may be efficiently transferred from one surface toanother.

During the manufacturing process it is also well known to twist, bend ortaper the boule depending on the end function desired. The taper, forexample, is made by heating the center and pulling the ends to producean hour-glass shaped boule with the fibers essentially parallel at thelarger diameter ends and smaller diameter center of the boule. Duringthis process the outermost fibers are stretched more and are longer thanthe innermost fibers. The boule is then cut in half at the smalldiameter center to provide two identical tapered halves, each of whichbecomes a fiber optic magnifier/minifier.

Faceplates serve as windows and transmit the image straight throughwithout changing the size or orientation. Twisted bundles function asimage inverters. Tapers serve as magnifiers or minifiers. The two endfaces of the bundles are preferably parallel planes and may be flat orcurved to a desired radius. It is well known to couple the small end ofthe taper to a self scanned array, such as, a charge coupled device("CCD") to convert the light level in a group of fibers or "pixels" to acorresponding electrical signal which can be digitized and reconstructedgraphically as an intensified image on a computer screen, for example inspectroscope, astronomical and medical applications.

Fiber optic bundles have found wide use in such fields as x-ray imageintensifiers and night vision goggles, for example.

As disclosed in U.S. Pat. No. 3,033,071, it is known to further heatsegments of the boule and pull the ends of the fibers to form a doubletapered, onion shaped boule which is then cut at a point in the taperedportion to form concave surfaces in one or both ends for use as an imageor field flattener. In this early device, however, the image is not of apoint which is the focal point of the bundle, normal to all fibers andwhich is a light or a scattered light source that radiates light suchthat the radiated light strikes the bundle at 0° incidence along theentire surface of the bundle.

As disclosed in U.S. Pat. No. 4,991,971, it is known to have a bundle ofequal length optical fibers each end of each of which is arranged in acircular array equidistant from the object being tested and the otherends of which are in a linear array whereby each fiber simultaneouslyreceives a different angular component of the scattered light at the oneend and transmits it to the other end such that the transmittedcomponents exit the linear array end simultaneously and are detected andconverted to electrical signals by a computer. The constraint of equallength fibers prevents use of a tapered fiber optic bundle in thissystem. Moreover, the device is limited to reading only that portion ofglobally scattered light that appears in the single plane in which thecircularly spaced fibers are located. Thus, the collection andcomputation of a scarlet profile for a spherical segment or a fullhemisphere requires rotating the sample as in other prior schemes withresultant lengthy, slow construction of the scatter profile.

These deficiencies are overcome in the present invention through the useof a tapered fiber optic bundle, each fiber element of which receiveslight normal to its aperture from the light source which is at the focalpoint of the bundle, and which has never before been used in measuringlight in a scatterometer. Moreover, a double tapered fiber optic bundleof applicant's unique design provides the remarkable advantages ofinstantaneous, spherically segmented or hemispherical collection oflight from a scatter source and has not been heretofore known.

c. Frame Grabber Algorithms

A frame grabber, or image memory, has been used in the past as part ofan image sensor processor. See, U.S. Pat. Nos. 5,040,116, 4,954,962 and4,843,565. Typically, the frame grabber is contained within a framegrabber pc-board, such as a type made by Coreco or Image Technologies,and is coupled to a data processing device, such as, an 80486 Intelmicroprocessor driven computer, which, accesses the image memory and,according to a predetermined algorithm, reconstructs the image on acathode ray tube or other luminescent screen. A standard frame grabberis capable of resolving 256 shades of grey. Where the ambient light hasto be eliminated or otherwise adversely influences the reconstruction ofthe subject image, it has also been known to use a technique ofsubtracting out the ambient light value from the data. See U.S. Pat. No.4,991,971 (4:56-64).

Prior frame grabber algorithms have been slow and inefficient and haverequired the use of expensive CCD cameras. The advantage of applicant'sunique algorithm is that its high level of efficiency enables the use ofa low cost charge injection device ("CID") camera which eliminates thesignificant "blooming" problem experienced with CCD cameras when pixelsbecome saturated and which prevents good scatter profile reconstruction.

SUMMARY OF THE INVENTION

Set forth below is a brief summary of the invention in order to achievethe forgoing and other benefits and advantages in accordance with thepurposes of the present invention as embodied and broadly describedherein.

One aspect of the invention is an apparatus and process for collectinglight which comprises a plurality of optical fibers the one ends of eachof which are fixed in spaced relation to each other, in a curved surfacehaving substantially a radius of curvature normal to each fiber and theother end of each of which is arranged in an indexed array, and thelongitudinal axis of each fiber at the one end substantially convergesat the common point, whereby light radiating angularly from the point isreceived simultaneously at the one end by each fiber and is transmittedto the other end.

Further features of this aspect of the invention include embodimentswherein the curved surface is a full hemisphere, a spherical segment ora linear segment.

A second aspect of the invention is an apparatus and process formeasuring light reflected from a surface including a housing, a laserdiode light source for illuminating the surface and supported within thehousing, a tapered fiber optic bundle supported within the housing andhaving a concave face on the tapered portion at one end, with the otherend formed as a flat array adapted for transmitting light reflected froma point on the surface, and a CID camera supported within the housingand having a scannable area array for receiving the transmitted lightbeams and converting the beams to electrical signals.

A further feature of this aspect of the invention is controlling thepower of the laser by controlling its on time.

A third aspect of the invention is an apparatus and process of acquiringwith an x-y scannable array camera light reflected by a subject that mayexceed the dynamic range of the camera including the steps of a)measuring the ambient light and storing the measurement in a referenceframe, b) illuminating the subject with a laser diode light source for apredetermined time period, c) collecting the light beams reflected fromthe subject during the on period and transmitting the beams to thearray, d) digitizing the collected data by x-y scanning the array andconverting the light beam to electrical data, e) storing the digitizeddata in the next frame, and f) repeating steps (b) to (e) whileincreasing the on time of the laser diode by predetermined amounts (forexample, one order of magnitude) during each repetition, whereby areference frame and N data frames are collected and stored in N+1sequential frames.

A fourth aspect of the invention is an apparatus and process ofreconstructing a single data profile from the data stored in theplurality of sequential x-y oriented memory frames of the computer framegrabber the first frame of which is a reference frame in which the datarepresents a factor common to all frames in the sequence, and theremaining frames of which are data frames including the steps of a)setting an x-y oriented profile array in memory and filling the arraywith zeros, b) computing a scale factor, c) subtracting the commonfactor data (i.e. ambient light) in the reference frame from the data ineach of the data frames in the sequence, d) scaling the data in thefirst data frame by the scale factor and adding the scaled data to theprofile array, e) scaling and adding the data stored in each x-y pointin the next succeeding data frame to its corresponding x-y location inthe profile array only if data has not been previously stored in thatlocation, and if (1) the data already in that x-y point in the array iszero, and (2) the data in that x-y point in the current and any priordata frame is less than T, where T is a threshold level representing alevel above which the data is invalid, and f) repeating step (e)separately in sequence for each subsequent data frame in the sequence.

A fifth aspect of the invention is a scatterometer apparatus and processfor measuring light reflected angularly from a point which includes apower controlled laser light source for illuminating the point, a fiberoptic bundle focused on the point for collecting light beams reflectedtherefrom, a camera for converting the reflected light beams intoelectrical signals, the signal level of the collected light beams beingraised above the dark current noise level of the camera by sequentialincreases in the on time of the laser by predetermined amounts to formsequential images stored in a frame grabber, and the frame grabber alsosequentially digitizing the stored images and reconstructing therefrom asingle scatter profile of the point while simultaneously scaling out theorder of magnitude increases.

A further feature of this aspect of the invention is that the scatterprofile may be completely reconstructed within one second.

A sixth aspect of the invention is the process of collecting light in ascatterometer to convert the light to electrical signals which includesthe step of directing the light into a tapered fiber optic bundle. Afurther feature of this aspect of the invention is directing the opticalfibers in the bundle such that they have a common field of view.

A seventh aspect of the invention is the method of manufacturing whichincludes forming a tapered fused fiber optic bundle and cutting aconcave face in the tapered portion normal to the bundle such that allof the fibers have a common field of view.

The principal advantages of the scatterometer of the present inventionare its compact size, ruggedness, speed, and hemispherical capability.

This new, table top, portable instrument is the fastest, most powerfulscatterometer on the market. It allows high resolution (0.125°) whichcan be increased by using higher density CID arrays, partial or fullmeasurement of hemispherical scatter data in less than a second with nomoving parts from a very small 12"×10"×6" footprint.

This new technology enables taking hemispherical, rather thancurvilinear, data measurements with negligible change in cost, whileproviding greatly increased performance benefits.

The fiber optic bundle scatterometer ("OMNISCATR"™) of the presentinvention has the ability to measure light scatter caused by everythingfrom scratches, blemishes, bubbles, subsurface defects, and surfaceroughness ("RMS") to BRDF AND BTDF. It measures over 200,000 points overthe hemisphere, two orders of magnitude more spatial data than any knownscatterometer, thus, enabling detection of defects regardless oforientation, and determination of the orientation itself.

Until now, scatterometers have been mainly used in the aerospaceindustry. The new generation scatterometer of the present invention,however, with its higher speed, smaller size and lower cost is availableas a quality control device to many other industries that requireassurance of high surface quality, such as, for computer screens,precision bearings, flat and power optics and specially coated orpainted surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view in partial section of the interior of thehousing showing the measurement head containing the laser source,collection optics, beam dump and camera assembly.

FIG. 2 is an optical schematic drawing showing focusing of the lasersource light rays on the beam dump and optical collector.

FIG. 3 is a schematic top plan view of the interior of the housingshowing the measurement head containing the laser source, collectionoptics, beam dump and fiber optic/camera assembly.

FIG. 4 is a plan view in partial section of the onion-shaped boule fromwhich the preferred embodiment of the double tapered fused fiber opticbundle of the present invention is made, and, further shows the radiusof curvature cut into the converging tapered area to form the concavefront face of the bundle.

FIG. 5 is a side elevational view of a double tapered fiber optic bundleof the present invention prior to its being cut into individual bundles.

FIG. 6 is side elevational view in cross section of a fiber optic bundleof the present invention using single fibers in the form of a fullhemisphere.

FIG. 7 is a perspective view of yet another alternative embodiment ofthe single, hemispherically cut, tapered fiber optic bundle of thepresent invention with the light source going through the bundle.

FIG. 7B is a possible front elevational view of FIG. 7.

FIG. 8 is a perspective view of yet another alternative embodiment ofthe single, hemispherically cut, tapered fused fiber optic bundle of thepresent invention made in by cutting and fusing together two separatehalves.

FIG. 9 is a schematic top plan view of an alternative embodiment of theinterior of the housing showing the source, collection optics, andcamera. Only a partial tapered fiber optic bundle is used the end ofwhich is imaged onto the detection array.

FIG. 10 is a side elevational view of an alternative embodiment of thefront half of the tapered bundle of FIG. 8 with a higher angled taperthan FIG. 11A.

FIG. 11A is a side elevational view of an alternative embodiment of thefront half of the tapered bundle of FIG. 8.

FIG. 11B is front elevational view of FIG. 11A.

FIG. 12A is a perspective view of yet another alternative embodiment ofa linear type fiber optic bundle of the present invention (fused orsingle fibers).

FIG. 12B is a plan view in partial section of the bundle shown in FIG.12A.

FIG. 12C is a further perspective view of the bundle shown in FIG. 12A.

FIG. 12D is a rear elevational view of small exit end of the bundleshown in FIG. 12A formed into a matrix for attachment to the CID cameraarray.

FIG. 12E is a plan view of the bundle shown in FIG. 12A.

FIG. 13 is a side elvational view of the fiber optic assembly unitshowing its mounting in its encapsulation fixture and spring basedattachment to the CID camera.

FIG. 14 is a top plan view in partial section of the measurement headshowing the Z-stage alignment adjustment mechanism.

FIG. 15 is a side elevational view in partial section of the measurementhead shown in FIG. 14 showing the azimuth alignment adjustmentmechanism.

FIG. 16 is a schematic block diagram of the scatterometer dataacquisition system which includes a computer, frame grabber, IEEE-488card, TTL card, and a laser diode controller.

FIG. 17 is a flow chart showing the algorithm for the data acquisitionprocess of the present invention.

FIG. 18 is a flow chart showing the algorithm for the datareconstruction process of the present invention.

FIG. 19 is a schematic tap plan view of the measurement layout forcalibration and system profile.

FIG. 20 is the computer screen showing the format for presenting theanalyzed data.

FIG. 21 is a spectral response curve for a CID camera of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT TABLE OF CONTENTS I.DEFINITIONS II. INTRODUCTION III. INSTRUMENT DESCRIPTION

A. Overview

B. Optical Layout

1. Source Section

a. Laser Diode

b. Cornu Psuedo--De Polarizer

c. Focusing Optics

2. Collection System

a. Tapered Fiber Fundle

(1) Unitary Double Tapered Fiber Optic Bundle

(2) Full Hemisphere Fiber Optic Bundle

(3) Fused Unitary Double Tapered Fiber Optic Bundle

(4) Partial Tapered Fiber Optic Bundle

(5) Linear Array Fiber Optic Bundle

b. Fiber Optic/Camera Assembly

c. Optical Filter

3. CID Camera

4. Enclosure & Adjustments

a. Housing and Mounting Plate

b. Compactness Of Measurement Head

c. Protective Shutter

d. Ruggedness Of Measurement Head

IV. ALIGNMENT

A. Internal Alignment

B. External Alignment

V. DATA ACQUISITION HARDWARE AND SOFTWARE

A. Computer Hardware Interface

B. Software User Interface

C. Data Acquisition and Data Reconstruction

1. Ambient Light Compensation

2. Scatter Measurement/Data Acquisition

3. Data Reconstruction

D. Data Display

E. Data Analysis

1. Normalization

2. Calibration

3. System Profile

4. Volume of Information

F. Data Storage

G. Two Dimensional Plotting

H. Three Dimensional Plotting

I. Graphic Output

VI. ADDITIONAL MEASUREMENT TECHNIQUES

A. Rapid Surface Scan

B. Processing and Surface Analysis

VII. TECHNICAL NOTES AND CALCULATIONS

TECHNICAL NOTES

Detector Linearity

Calibration

RSS Calibration Method

ABDM Calibration Method

Stray Light Control

CALCULATIONS

    ______________________________________                                        DEFINITIONS                                                                   TERM     MEANING                                                              ______________________________________                                        BRDF     Bidirectional Reflective Distribution Function                       BTDF     Bidirectional Transmission distribution Function                     BSDF     Bidirectional Scatter Distribution Function                          TIS      Total Integrated Scatter                                             RSS      Reference Sample Substitution method                                 RMS      Surface Roughness                                                    PSD      Power Spectral Density                                               ABDM     Attenuated Input Beam Direct Measurement                             HgCdTe   Mercury Cadmium Telluride                                            NIST     National Institute of Standards & Technology                         FASCAT   Fully Automated Scatterometer                                        Sample   Detector which intercepts scatter from the                           Detector sample                                                               Reference                                                                              Detector which monitors laser fluctuations                           Detector and attenuation                                                      SNR      Signal-to-Noise Ratio                                                LVND     Linear Variable Neutral Density                                      AR       Anti-Reflective                                                      CCD      Charge Coupled Device                                                CID      Charge Injection Device                                              TTL      Transistor To Transistor Logic                                       Source   The laser radiation source with beam shaping                         Head     and collimation in an enclosed compact unit.                         Collection                                                                             All collection optics and the detector on                            Head     a small base plate.                                                  Measurement                                                                            An enclosed unit to be mounted on a                                  head     production machine containing the source                                      head, collection head, external focus                                         control, shutter, and mounting                                                accommodations.                                                      ______________________________________                                    

II. INTRODUCTION

The detailed description of the invention has been divided into severalmajor sections. The first section, Instrument Description, contains abrief overview and describes the functional components of thescatterometer. The next section describes the internal and externalalignment of the measurement head. The data acquisition and softwaresection describe the control of the scatterometer, the analysis of thedata and, by its nature, the use of the instrument.

III. INSTRUMENT DESCRIPTION

A. Overview

An overview of the measurement unit 10 and the data acquisition andreconstruction 200 system of the new scatterometer design are shown inFIG. 1 and 16, respectively. The scatterometer of the present inventionincorporates two uniquely distinctive concepts. The first is theincorporation of a tapered fiber optic bundle 40 (FIGS. 2 and 4) tocollect the scattered light from the sample 65. The second is a dataacquisition algorithm 170, 180 (FIGS. 17, 18) that allows rapidcollection of the five orders of magnitude of light required withoutusing the traditional slow detector/lock-in amplifier pair techniqueused in current scatterometers. Some advantages offered by this designinclude:

Hemispherical data acquisition in less than 1 second

Resolutions of up to 0.125° or higher

Measurement of in-plane and out-of-plane scatter

Using a collection of many fibers 44 (FIGS. 2, 4) fused together with aconcave face 41 cut into the tapered section 48 of the bundle, scatterdata can be collected and directed to a camera 130 for very fast datacollection. This aspect of the invention allows all of the data to becollected simultaneously, eliminating many undesired repetitiousmeasurements based on time related variables. To get the low signallevels required for at least 10⁻⁵ BRDF, the laser power and/or cameraintegration time will be controlled to raise the signal level above thedark current noise in the camera. A method 170 has been developed tocontrol the laser power so that the data can be collected approximatelyone order of magnitude at a time. With a conventional frame grabberdigitizing multiple camera images, each order of magnitude of data canbe collected separately and used 180 to reconstruct the scatter profile.At 30 frames/second collection rates, the time to collect the data iswell under a second.

The detailed description of the instrument section of the scatterometeris further divided into several sections to facilitate a complete systemdescription. These sections are optical layout and specificationsummary, source optics 20, collection optics 30, CID camera 130, and thesystem enclosure and adjustments 140.

Working examples of the scatterometer are set forth in Table 1 whichsets forth a component summary of the measurement head characteristics,Table 2 which shows the instrument's overall parameters, and Table 3which describes the instruments data acquisition and softwarecharacteristics. More technical notes such as stray light control andCID camera calculations are included in Section VII below.

B. Optical Layout

The optical layout is shown in FIGS. 1, 2, 3, 4 and 13. As best seen inFIG. 1, the optical layout includes the laser diode 21 and controller22, fold mirror 24, psuedo-depolarizer 25, focusing optics 26, shutterassembly 143, beam dump 31, and fiber optic/camera and camera controllerassembly 30 with camera imaging array 131 (FIG. 7).

1. Source Section

a. Laser Diode

To obtain ample power in a small package a conventional laser diode 21is used as the laser source. The laser diode 21 is powered andcontrolled within the housing 141 by the laser connector 22. The laserbeam is directed to and reflected by a fold mirror 24 supported in thehousing 141 at an angle.

The particular diode and power chosen is based on the minimum BRDF of10⁻⁵ sr⁻¹, optimal reception characteristics of the collection optics,and the camera sensitivity. There is a trade off in selecting laserdiode power and wavelength. Visible camera sensitivity is at a peakaround 670 nm, while diode power peaks around 840 nm or higher. As shownin the power calculations, the laser power and wavelength are optimal at70 mW and 840 nm respectively. The diode unit 21 is complete with beamcircularization (beam forming) and a collimated output beam of 7.5 mm.The diode unit 21 includes a driver (not shown) for power stabilization,thermoelectric cooling, and TTL/Analog modulation. The cooling isrequired since power output, which must be held constant, is temperaturedependent. TTL modulation 160 is used to pulse the laser diode 21 orturn it on for a specified time, thus controlling the amount of power onthe sample 65 and detector array 131. The lifetime of the laser diode 21is about 50,000 hours.

In the preferred embodiment the laser diode controller controls theamount of power on the camera to five orders of magnitude. Laser diodesare typically only adjustable in power level over one to one and onehalf order of magnitude. So the alternative is to control the time thatthe power is incident on the camera. The controller 23 is set tomodulate the time the laser is on from a range of 10 ns to continuouslyon. This capability yields several orders of magnitude of BRDF rangeabove what is required. The rise-fall time of the laser power output isalso in the 10 ns range. This range is sufficient to allow control ofthe incident laser energy over more than the required five orders ofmagnitude. The laser timing is controlled through the use of a TTL 160level initiate signal provided by the computer system 150 and loadingthe controller 23 with the desired power level through an IEEE-488interface 159.

The output power per pulse is controllable to a maximum specified forthe laser. The controller 23 has the ability, through the use of aphotodiode feedback loop (not shown), to both stabilize and measure thepower output.

Power and timing calculations are provided in Appendix B.

A working example of a laser diode unit 21 is a Model 06 PLL 807 made byMelles Griote and has the following characteristics:

833 nm beam wavelength (required visible to 3.0 microns)

70 mw power output of the laser unit

7.5 mm collimated circular beam (internal beam shaping) beam profile isR/e² =3.4 mm

3 part collimating lens--spherical aberration correction and collimation

cylindrical lens--astigmatism correction

anamorphic prisms--circular beam shaping

Thermoelectrically cooled

External power supply/controller Melles Griote MOdel 103 allows fullcontrol of laser power, temperaturel and on/off time

Pulse modulation up to 1.0 MHz (used to control power output)

b. Cornu Pseudo-depolarizer

Due to the use of a laser diode 21 in the measurement head 10, thesource section 20 starts with linearly polarized light. Since no truedepolarizers exist, the laser light is spatially randomly "depolarized"through the use of a cornu pseudo-depolarizer 25. Careful attention tothe beam size and intensity pattern is required to achieve a high degreeof depolarization. Depolarization efficiencies of better than 95% areattained which is sufficient to achieve the desired accuracy of BRDF.

It is sometimes useful to use polarized light, but in general BRDFmeasurements should be taken with unpolarized light, randomly circularpolarized light, or, if need be, circular polarized light. Relatingmathematically the s or p polarization sources to the scatter and thesurface characteristics, and then compensating for the polarizationeffects in software is a complex problem. To avoid this problem apseudo-depolarized laser beam is used for the measurements. activity incrystalline quartz. It operates on collimated beams of light,transforming linear polarization states into a complicated and spatiallychanging continuum of linear polarization states.

c. Focusing Optics

As shown in FIG. 1, the output of the laser/depolarizer assembly isdirected onto the sample and focused on the collection optics withfocusing lenses which function as an inverted telephoto system, i.e. anegative and positive lens system.

The lens system 26 is designed to minimize the collection aberrationsfrom the negative lens 27 and positive lens 28, each of which has anonreflective coating.

The lenses are a good quality achromat and are adjustable to control thefocus of the beam with respect to the fiber optic bundle 40. Theadjustment of the focus lenses 27, 28 allows for proper compensation forpowered optical surfaces. However, the range of the adjustment may berestricted by the small size of the measurement head.

Focus adjustment is performed manually by a micrometer 29. Approximately3 mm of micrometer travel is required for infinite (flat surface) to 600mm (concave surface) radius of curvature. The focusing optics 26 produceapproximately a 4 mm Gaussian spot size on the sample. Diffracted energyfrom the test sample not striking the fiber bundle is kept well within2°.

2. Collection System

As best shown in FIGS. 1, 3, 4 & 11 the collection optics 30 include atapered fiber optic bundle and a camera. The tapered fiber bundle whichcollects the scattered light is directly connected to a solid state, CIDcamera.

a. Tapered Fiber Bundle

There are several embodiments for the fiber optic bundle of the presentinvention.

(1) Unitary Double Tapered Fiber Optic Bundle

As seen in FIG. 4, the preferred embodiment is a one piece doubletapered fiber optic bundle 40 with a concave face 41 cut into thetapered portion 48 at one end. Prior to the spherical cut forming thefront face, the boule is shaped like an onion. The fiber bundle is asingle piece of glass made of fused 25 um diameter fibers 44 tapereddown to 3.8 um diameter by a well known process of heating, stretchingand cutting. The bundle 40 provides approximately 40% transmittance. Theradius of curvature R on the front face 41 is required so that allfibers 44 have a common point of view and are, thus, viewing the samepoint 43 on the sample. This construction provides the enhancedfunctionality of partial or full hemispherical measurements depending onthe type of fiber optic bundle used and the way in which the sourcelight is applied.

In the preferred form of the invention, a unitary double tapered fiberoptic bundle 40, the bundle is stretched a first time to produce thewell known hour-glass shape having a narrow portion in its center withwide portions at its ends as seen at 40a in FIG. 5. Then, each of thewide ends is stretched a second time to produce the new onion shapehaving a narrow section at each end and a wide section in the middle asseen at 40b in FIG. 5. Now, when the bundle is cut at the narrowsections 40c, 40d, what is left is a boule 40 which looks like an onionas shown in FIG. 4 in that the wide portion 47 of the taper is in thecenter and the narrow portions 45, 46 of the taper are at each end. Thespherical cut 41 is then placed at a point in the tapered portion 43near the wide center 47 where the fibers 44 are converging to form theunitary double tapered shape 40 shown in FIG. 4. As further seen indotted lines in FIG. 5, spherical cuts 41 may be placed at other desiredlocations on the taper where the fibers are converging or diverging. Theend 49 is cut in a flat planar surface forming an indexed array adaptedto be mated to a camera for converting the transmitted light toelectrical signals.

As shown in FIGS. 4, 10, 11A and 13, the desired curvature is obtainedautomatically from the tapered fiber optic bundle by cutting the face 41at a selected radius R at the converging point 48 of the taper to form aspherical surface in the tapered bundle where the individual fibers areconverging. The cut is made roughly perpendicular to each fiber 44 inthe bundle 40 regardless of its position. The preferred radius on a 2"diameter boule gives a 40° to 60° three-dimensional cone anglemeasurement range of the light scatter from the spot on the sample whichis the focal point of the concave surface 41. All individual fibers 44within the bundle have a common light source 43 field of view. Thelongitudinal axis 43a (FIG. 6) of each fiber 44 is normal to the cut ofthe bundle 40. In addition each incident ray 44a (FIG. 7A) of lightemanating from the focal point of the bundle and striking a fiber isnormal to the cut of the bundle at the point it enters the fiber. Whileeach fiber's surface normal may actually hit the sample surface atvarious locations within the illuminated spot, nevertheless, the effectof this slight deviation is averaged out, since each fiber 44 collectsthe scatter of the entire illuminated area only at its viewing angle.

A working example for FIG. 11A is as follows:

Outside diameter of bundle=2"

Inside diameter at cut point 48=1.75"

R=2.12"

d=47°

A narrow (830 nm) waveband anti-reflective filter coating can be put onthe back face 49 or front face 41 of the fiber bundle 40 which is verydurable. This reduces unwanted light of other wavelengths.

Some advantages of the fiber bundle 40 are that a high density of cameradetector elements may be mated to the small end of the taper (minifierfunction) and three dimensional out-of-plane measurements may be takenat high speed.

Stray light reflections from the faces of the fiber bundle 40 itself arecontrolled by the use of a good quality AR coatings. The specularreflection off of the polished AR coated face of the fiber bundle wouldcreate a scatter signal that would exceed the expected measured signalby orders of magnitude at the far angles. Even a black spot painted onthe fiber bundle at the specular position would create appreciable straylight. For a specular measurement the light trap needs to be asophisticated combination of a specular black surface and a diffuseabsorber. For this reason the specular beam is excluded and a beam dump31 for the specular beam has been included.

Currently the collection optics can be swung into the specular positionand out again for separate specular and low scatter measurements. Futureversions of the fiber shell may allow simultaneous measurement of nearspecular and far angles (low scatter) if a good solution is found forsuppressing the specular beam.

The collection system 30 is susceptible to ambient light. Ambient lightis controlled through the use of a narrow band filter. It is recommendedthat fluorescent lights be used in the measurement room, since they havelower emission in the 0.670 μm to 0.900 μm region, which is an operatingwavelength region of the scatterometer. Any remaining ambient light upto a certain threshold can be subtracted out of the data after it isacquired as discussed below in the data acquisition section.

Analysis results have shown a worst case stray light condition of oneorder of magnitude above the desired scatter level when the specularbeam is incident on the fiber bundle. When the specular reflection isdirected into the beam dump no stray light problems have beenencountered.

(2) Full Hemisphere Fiber Optic Bundle

Shown in FIG. 6 is yet another embodiment of the fiber optic bundle ofthe present invention. In this form of the invention a hemisphericaldome 32 which is one half of a full sphere is made of any rigid materialand is pre-drilled with a plurality of radial holes 33 the common fieldof view of which is the center of the sphere 43. A fiber optic 44 isinserted and secured within each radial hole 33. The fibers are thenbundled together and the loose ends are arranged into an array 95 suchas is shown in FIG. 12D. Any one of fibers 44 may function as a conduitfor the laser diode 21 source light to illuminate the point 43 on sample65. Alternatively, a fiber 44 may be removed and replaced by a conduitfor the light from laser diode 21. If stray light is not a problem, thespecular beam is used for measurements. If stray light is a problem, ahole may be drilled in dome 32 allowing the specular beam to passthrough the dome thereby essentially eliminating the stray light. Thisform of the invention is used by placing the hemi-dome 32 over the pointon the surface to be measured and illuminating the surface with thelaser diode source 21. Full simultaneous hemispherical measurements oflight scattered by the point sample are enabled because all fibers in afull hemisphere have a common field of view at that point.

(3) Modified Fused Unitary Double Tapered Fiber Optic Bundle

A further embodiment of the double tapered fiber optic bundle 60 isshown in FIG. 7A. In this version of the bundle, the laser source beamsare directed onto the sample through an enlarged fiber 61 or through alongitudinal hole in the fiber which begins at the small end 62 orthrough a side wall 63 of the bundle and terminates on the sphericalface 64 cut into the converging portion of the large end of the bundle60. In this form of the invention, the laser source beam is transmittedto the sample 65 through the fiber bundle 60 and the specular beam 66 isreflected to a black spot 67 painted on the spherical face. Partialhemispherical measurements may be simultaneously made of the scatteredlight from the light source 43 which are then carried and reflectedthrough the fibers 44 to the small end 62 of the taper 60 whichterminates in an indexed array as shown in FIG. 7A and is adapted to bemated to a CID array camera 130. The light reaching the small end of thetaper is converted to electrical signals through the CID array 131 inthe camera 130 and then is digitized in the frame grabber 152 anddisplayed by the computer 150.

Still another embodiment for the fiber optic bundle 70 of the presentinvention is shown in FIG. 8. In this embodiment the fiber optic bundle70 comprises two-halves 71, 73. One-half 71 is a standard fiber opticbundle with flat faces at both ends. The other half is also a standardfiber optic bundle which has a hemispherical cut 74 in its front face inthe converging portion of the taper. The other face (wide end) 75 isflat. The two flat faces 75 at the wide ends of the tapers are rotated45° relative to each other and are then bonded together usingconventional bonding material to form a unitary double fiber optictaper. The purpose of the rotation is to eliminate any moire effect.

(4) Partial Tapered Fiber Optic Bundle

Shown in FIG. 9 is an alternate embodiment of the collection system 30.In this approach only a portion 73 (one half as shown in FIGS. 10 and11A) of the tapered fiber optic bundle is used, and an imaging system isused to focus the light onto the camera detector array 131.

In this embodiment the AR coated focusing lens 77 has a diameter largerthan the outside diameter of fiber bundle 73 to collect all of the lightemitted from the opposite end of the fiber bundle and focus it on thecamera detector array 131 in camera 130. Since there are many morefibers 44 than there are pixels it is possible to move the camera in andout to focus on portions of the image, thus achieving even higherresolution measurements, possibly up to 0.05°.

(5) Linear Array Fiber Optic Bundle

Shown in FIGS. 12A, B, C, D and E are several views of yet anotherembodiment of the fiber optic bundle of the present invention. In thesefigures is shown a linear array type fiber optic bundle 90. As shown inFIG. 12A, a working example includes 133 400 micron diameter fibers 44orientated in the aperture blocks 92, 93 which have been pre-formed forthe fibers 44 to be inserted at 0.25° spacings such that the 133 totalfibers 44 cover 33° field of view directed at a single point 43 (notshown) on the sample. The assembly is then bonded by potting 94 compoundwhich adheres the fibers to the aperture blocks as shown in FIGS. 12Band 12C.

The other end of the fibers 44 are brought together in an array 95 ofsmaller but thicker dimension as shown in FIG. 12D which can be readilymated to the CID camera. A working example of the small end array 95 isapproximately 6 millimeters wide and 4 millimeters high and comprises 8rows of 15 fibers, numbers 1 96 to 120 97, with the bottom rowcontaining 13 fibers, numbers 121 98 through 133 99. As a furtherworking example shown in the plan view FIG. 12E, the central axis 100 ofthe linear array 90 of the fibers at its wide end is orientated f=28 to29 degrees from the central axis of the small end of the fibers. Thefront face 101 of the linear array 90 is flat and perpendicular to thecentral axis of its plane of curvature.

b. Fiber Optic/Camera Assembly

Finally, as shown in FIG. 13, the double tapered fiber optic bundle 40of the present invention is seen in its encapsulated form in its housingattached to the camera head unit 130 which forms the fiber optic cameraassembly 110. The mated CID camera and fiber optic bundle assembly 110allow 90% transmittance. A fiber optic face plate 118 replaces thestandard glass window used in most arrays. The small tapered end of thebundle 40 is spring loaded 115 against the fiber optic face plate 118 inthe camera head unit. The camera head unit 130 is fixed to the housing112 which receives the fiber optic taper 40 and is mounted thereto by amounting plate 114. The fiber optic taper is located within theencapsulation fixture 111 within the housing 112 with potting 113filling the vacant areas and providing a seat for the fiber optic taper.A spring 115 holds the encapsulation fixture firmly against the fiberoptic face plate by having its other end abutting the spring retainer116.

c. Optical Filter

An optical filter 120 (FIG. 9) is selected to match the sourcewavelength which can be coated on the front or back surface of a taperor be a separate filter in front of the camera or collection optics.This eliminates a large portion of any stray ambient light.

3. CID Camera

The measurement requirements place unusually high demands on thedetection electronics. A single element detector is not feasible due tothe required resolution and measurement time. The mechanicalrequirements for scanning the device are also prohibitive. Linear arraydetectors are readily available, but when compared to area arraydetectors they are just as expensive and require less convenient supportelectronics. An area array camera is selected for data acquisition asthe best cost performance alternative since it is sold in a morecompetitive market, is available in a wide variety of formats, andprovides larger data capacity.

An area array CCD camera based on raw BRDF data requirements isavailable to measure and convert light beams to electrical signals. Sucha camera has the sensitivity and dynamic range required to measure aBRDF range of 10⁻¹ down to 10⁻⁵ Sr⁻¹, but its cost is prohibitive ($30k)In the preferred embodiment, however, the present invention uses a CIDanti-blooming camera 130 (which is an order of magnitude lower in cost)to collect the data in a unique process involving collecting the datarange segments (see software description for details). The scatter rangesegment and number of segments needed to reconstruct the data isdetermined by the particular CID camera specifications. The superioranti-blooming characteristics of the CID camera allows pixels to besaturated without affecting neighboring pixels. This tolerance isrequired in the unique reconstruction algorithm used in the presentinvention in which, during the data acquisition process, portions of theimage are allowed to saturate in sequentially collected range segments.

The appropriate laser power and camera features of variable timeintegration and anti-blooming allow the desired data to be collected inless than a second. As stated above, a standard fiber optic acts as aface plate interface between the small end of the fiber optic bundle andthe front end of the detector array.

A working example of a CID camera 130 is Model CIDTEC 2250 made byCIDTEC. This camera provides 512 pixels by 512 pixels resolution whichprovides 0.125 degrees angular resolution. Each pixel is a 15 um×15 umsquare. The camera is impervious to magnetic fields, shock, andvibration and does not degrade in sensitivity over time. Included in thecapability of the camera is asynchronous full-frame capture and multipleframe integration. The camera power supply allows full control of cameraspeed and integration time.

4. Enclosure & Adjustments

a. Housing And Mounting Plate

As shown in FIGS. 14 and 15, a strong, anodized, aluminum housing 141 isused to house all of the optics and electronics (except the computer).The measurement head housing 141 sits on top of a base mounting plate142 or platform which is adapted for mounting onto a production linemachine (not shown) and which contains the adjustment mechanismsrequired to perform the alignment procedure. These adjustments willinclude an axial position control 144 and an azimuth rotation control145.

A working example is as follows:

The actual size of the measurement unit housing 141 is 11.50"long×9.125" wide×5.25" high, including the base mounting plate 142 andadjustment mechanisms 144, 145. With the adjustment stages in operatingposition, the dimensions are 16.00" long×11.00" wide×9.50 high. With theadjustment stages fully extended, the dimensions are 22.00" long×11.00"wide×9.50" high.

b. Compactness of Measurement Head

The measurement head 10 (FIG. 1) itself is 9.5" long×8.5" wide×4.0"thick and contains the laser source 21, source optics 20, collectionoptics 30, and detector. These subsystems are integrated into a robustaluminum housing which is designed to withstand vibrations and G loadsof harsh working environments. The measurement head 10 is a fullyintegrated unit that may also be hermetically sealed as described below.The adjustments required for sample to measurement head alignmentrequire about 2-3 inches additional height.

c. Protective Shutter

The unit comes with a protective shutter 143 (FIG. 1), which is manuallyoperated in the base instrument. This function may be automated as anoptional feature. The shutter protects the internal optics from outsidecontaminants, especially when lubricants or other contaminants arepresent. If the shutter 143 is open and the sample under test is inharsh environments, applicant recommends precautions be made so thatcontaminants do not get onto the optics.

Alternatively a permanent window may be substituted for the shutter 143to protect the collection optics which makes the measurement head ahermetically sealed unit. If a window is used, the measurement head 10could actually be hermetically sealed and would become robust in thepresence of cutting oils and other contaminates. However, the effects ofsuch contaminants will strongly affect the measurements, particularlywhen sprayed onto the window.

d. Ruggedness Of The Measurement Head

The measurement head 10 contains very few individual parts; theseinclude the beam dump 31, diode head 21, source focus lens 26, fusedfiber bundle 40, collection lens 77, optical filter 120, and camera 130.These are mounted using vibration resistent mounts. Based on 0.5"aluminum, the measurement head will be built to withstand the vibrationsof 0.2 g and 5 khz. The only moving part inside of the measurement headis a focus control 29. Thus the lifetime of the instrument isexceedingly long, and it will require little or no maintenance otherthan occasional cleaning (environment dependent). The ruggedness is alsohighly desirable for many of the market environments anticipated for theinstrument's use (factory floor, aircraft, etc.).

IV. ALIGNMENT

A. Internal Alignment

The internal alignment of the scatterometer head is fairly simple.Internal alignment is required during assembly only, and is not requiredduring use. The internal alignment of the measurement head is inherentlydefined by the mechanics of the system. Each of the subassemblies hassufficient individual alignment control.

The source optics, including the laser diode 21 with heat sink, the foldmirror 24 mount, the pseudo-depolarizer 25 mount and the focus lens 26mount may be provided with their own internal tip and tilt adjustmentsrelative to the collection optics. In this case, the internal alignmentprocedure is very simple. The collection optics 30 will be orientedwithin the measurement head 10. A kinematically mounted reference sample146 (FIG. 19) will be used to define the desired test sample location.The source subassembly is first internally aligned to the referencesample 146 and beam dump 31.

B. External Alignment

The external alignment of the scatterometer to the part under test, orconversely, the alignment of the part under test to the scatterometer,is very simple. The alignment procedure is a two step process performedwhile watching a video screen. The user first controls the axialposition (z-translation) of the part relative to the instrument and thencontrols the azimuth (which sets the incidence angle of the measurementhead normal to the surface being measured) of the instrument relative tothe point under test. Only a small azimuth adjustment (<10°) isrequired. (FIG. 3 shows a=b=7°) Future expansion and automation of thisadjustment may be developed to perform surface scans.

Axial Adjustment: The axial position of the measurement head 10 isadjusted to maintain a specified distance from the test sample. Oncethis is done, use the angular control 147 (FIG. 1) to bring the specularbeam from the laser into the fiber optic bundle for the following twoadjustments.

Rotational Adjustment: After the axial alignment is performed, theazimuth lock 148 is unlocked (FIG. 1) and the measurement head 10 isrotated until the specular reflection of the laser source is aligned toand blocked by the beam dump 31. Again, this alignment is made whilewatching the video monitor 161 until the specular reflection appears ata specific location on the monitor and takes only a few seconds.

Focus Adjustment: After rotational adjustment the focus control 29(FIG. 1) is used to compensate for powered optics by getting the spot onthe monitor as small as possible. The collection optics are then againmoved to the measurement position by using angular control 147.

V. DATA ACQUISITION HARDWARE AND SOFTWARE

The software section of the detailed description is divided into severalsections which include computer hardware interface, software userinterface, data acquisition and reconstruction, data display, dataanalysis, data storage, two dimensional plotting, three dimensionalplotting, graphical output and measurement speed.

A. Computer Hardware Interface

A computer system 150 is used as the measurement controller. Thecomputer-scatterometer data acquisition system is shown schematically inFIG. 16. The computer interface allows for complete automation of thedata acquisition process, such as laser power control, cameraintegration time, ambient light monitoring, and data capture. Theautomation provides consistency between measurements and reduces chancesfor human error. Also, the hardware eases labor intensive tasks such asfocusing and diagnostic checking by providing real time video and statusinformation. In addition, the features of the fiber bundle are fullyutilized to provide a set of powerful capabilities such as realtimevideo monitoring, higher density measurements 0.125° or greater andmeasuring a large portion of the scatter hemisphere. These state-of-artcapabilities use low cost readily available hardware. The design of theacquisition system has no moving parts, yet acquires large amounts ofdata in less than a second.

The recommended computer is a 486 compatible PC 148 which has thecapability to interface to all of the scatterometer components andrequires one IEEE-488 card 149 for diode monitoring and a TTL card 160for high speed diode modulation.

A working example of a PC computer system is as follows:

Gateway 486DX2, 50 MHz/SMB for

high speed display of data, and

high speed pulse modulation of laser

640KB of RAM memory is sufficient for 50 sets of in-plane measurementswithout storage to disk. Hemispherical data requires storage to disk foreach data set.

1.2 MB Floppy

1.44 MB 3.5" floppy for data transfer

MS-DOS 5.0

Windows 3.1

200 MB Hard Drive for mass data storage

VGA Graphics 800×600×256 for graphics display of data

Multisync Monitor

Frame Grabber to capture the video data from the camera

IEEE-488 Card & cable for data transfer to/from the laser power supply

TTL Digital Interface Card & cable for high speed pulse modulation oflaser

Hybrid Data Acquisition/Control & Analysis Software

Abaton Laser Printer (optional)

Print-A-Plot HPGL Laser converter (optional)

The software and hardware developed will be compatible with 286/386systems. However, the data display rate on these systems may take morethan one second.

B. Software User Interface

The user interface will be a Graphical User Interface (GUI) with thelook and feel of Windows 3.0, but will not require Windows 3.0. Highspeed graphics will be used to display information on the screen in nearreal time (less than 2 seconds). Data entry will be by keyboard 153 andmouse 154. Graphical Icons, axis scales, slide bars, push buttons, dataentry windows with realtime error checking, and drop down menus will beused to provide intuitive and flexible interaction with the acquisitionand data visualization process.

C. Data Acquisition and Data Reconstruction

As shown in FIG. 16 the acquisition system is composed of laser diode 21and controller 155, a fiber optic/CID camera assembly 110 and controller132, a 486 personal computer 148 and software. As far as the user isconcerned, the measurement process consists of entering measurementparameters and then initiating the measurement sequence. The automaticmeasurement process consists of an ambient light test, scattermeasurement, and data reconstruction. Once the data has beenreconstructed it is displayed in the format previously specified by theuser. The measurement and display process can be repeated once, aspecified number of times, or continuously at a user selected interval(three seconds and up). After the data is collected it can be analyzedand stored. Detailed descriptions of these functions are in thefollowing subsections.

1. Ambient Light Compensation

After all parameters have been selected by the user and before each andevery scatter measurement the ambient light level will be measured. Thismeasurement (laser off) will be continuous with a warning light on thecomputer screen until either the user aborts or the light level isdropped below an acceptable level. If the ambient measurement isterminated due to the light level being dropped to an acceptable levelthe scatter measurement will commence. If the ambient level is initiallyin the acceptable range, then the ambient and scatter measurements willboth be accomplished in less than one second. Any mean ambient lightwithin the acceptable threshold will automatically be subtracted out ofthe data.

2. Scatter Measurement/Data Acquisition

A camera and frame grabber which digitizes the video image will be usedto collect the data. To collect data beyond the range of the camera,applicant has developed a unique algorithm to place multiple orders ofmagnitude of data one per frame (camera limit) in separate memory pagesor frames on the frame grabber. The first page (frame) to be acquiredhas the power (φ₁) and time (t₁) adjusted such that no pixels of theimage are saturated. Each of the successive pages (frames) have thelaser power incremented by an order of magnitude. Eventually some pixelsclose to the specular beam will saturate (without damage). Thissaturation is detectable and ignored since the relevant measurements forthe saturated pixels were saved on a previous page (frame) when theywere not saturated. The camera is anti-blooming, so saturating pixels donot offset neighboring pixels.

The detailed data acquisition process 170 steps shown in FIG. 17 aredescribed as follows (variables are in italics):

    ______________________________________                                        Step Description                                                              ______________________________________                                        1.   Set variables a, b and p to start levels 171, where a                         and b represent predetermined exponential powers (for                         example, powers of ten) of on time of the laser as a                          function of a, b and p as follows: source-on-time =                           t = t.sub.1 × 10.sup.(p-1)(a-b) where t.sub.1 is the initial            time for page                                                                 1 such that no pixels are saturated given the laser                           power and p represents page number of the frame-                              grabber. As a working example, set a = -2; b = -1;                            page = 0;                                                                2.   With the laser off measure ambient light and store in                         the first page of the frame grabber 172. (Page = 0).                     3.   increment 173 page = page + 1.                                           4.   Turn the laser on for sufficient time for the camera                          to collect data between 10.sup.a to 10.sup.b in magnitude 174,                and, then                                                                5.   Turn laser off 174.                                                      6.   Digitize image and store the data in the current page                         of the frame grabber 175.                                                7.   Decrement variables a, b by 1 at 176, a = a - 1;                              b = b - 1; (I.e., increase the diode on time by a                             predetermined amount (for example, one order of                               magnitude)                                                               8.   Repeat steps 3 through 7N times 177.                                     ______________________________________                                    

3. Data Reconstruction

The frame grabber now has N+1 images (data magnitude range dependent)stored on it, one per frame (page). The first image contains the ambientlight measurement, which is a factor common to all of the frames in thesequence, and which is subtracted out of all pages leaving the reducedvalues in the remaining pages which comprise the scatter measurementdata otherwise unchanged. These remaining frames of data containinformation such as saturation, data not yet detectable, and greylevel.From this information a single scatter image can be constructed thatrepresents the scatter profile. During the reconstruction process thisscatter profile is normalized (scaled) relative, to the varied incidentpower of the laser diode and relative to a systematic referencecalibration measurement taken previously. Each frame can hold variouspixel resolutions of information depending on the angular resolutionchosen. Only the measurement range selected by the user will beprocessed. In this way the user can choose between less data at higherdisplay speeds or high volume information at lower display rates.

Once the N+1 data frames, i.e. one ambient reference frame and N dataframes, have been collected, the detailed steps of the datareconstruction process 180 to reconstruct the scatter profile are shownin FIG. 18:

1. A profile array (RDATA) is set up in computer memory filled withzeros 181.

2. A dual function scale factor is computed. The scale factor is afunction of the source on time of the laser diode and a function of thesystematic reference calibration measurement. As seen below, preferablyboth scale factors are applied simultaneously during the reconstructionprocess thereby shortening the reconstruction time period. However,optionally, the systematic calibration scale factor may be applied afterthe reconstruction is first completed.

3. The ambient light (reference frame, page 0) is subtracted from thescatter data 182. This is assumed to be the same for all frames ofscatter data since the camera integrated for the same time on eachframe. It was the laser on-off time that changed.

4. Set variable p=1 at 183 and scale the data in the first data frame(page 1) and add the scaled data to the profile array RDATA at 184.

5. Scale and add the scatter data in the next succeeding data frame tothe profile array according to the formula described below and showngraphically in FIG. 18 at 185-190. This is done on a pixel by pixelbasis 185, 186, 187, 188, 189 and 190, and for each data frame insequence 191, storing the pixel data only if the pixel data has not beenstored previously 188. That is, the pixel data from the data frame isstored in its spot in the profile array only if the data already in thatspot in the profile array is zero and the data in that spot in thecurrent and any prior data frame is less than the saturation level ofthe pixel. The reason for this double test as to saturation level is tonegate the effect of oversaturation of a pixel which causes the pixel toreverse color and look like it is not saturated. Looking to the priorframe to confirm the pixel was saturated prevents the frame grabber frombeing fooled by the pixel color reversal due to oversaturation. Thus, ifpixels in the current frame or previous frame are saturated or anypixels in any previous frame (not the current page) are non-zero thenthe pixel has been stored previously 188 and data in the current frameis ignored. Thus, data gets added to an x-y location in the profilearray only once in the entire reconstruction process. If pixels in theprofile array are still zero, it means the scatter was too low tomeasure.

6. Step 5 is repeated for all frames until all frames 191, 185 have beenprocessed.

Now that the scatter profile has been constructed it can be displayedand/or stored to disk.

D. Data Display

The latest GUI technology, and high speed graphics are used to visualizethe data as shown in the graphical output screen 193 in FIG. 20. Usinggraphical slide bars the user is able to select multiple segments (near,mid, far angles) of the BRDF curve. The values over the range of asegment can then be reported by peak value and/or average value. Thesevalues can be displayed in one window while the data is plotted inmultiple formats in another window. Other values that can be displayedare: diode output power, average ambient light level, RMS, PSD, TIS, andauto-correlation. Also the camera system provides real time display oflarge sections of the scatter hemisphere.

E. Data Analysis

1. Normalization

The data is normalized to the laser output power. This factor isreflected in the scale factor in block 181 of FIG. 18. The diodecontroller 155 has a photodiode feedback circuit (not shown) thatstabilizes the output power of the laser. This circuit can be sampled toread out the power output from the laser. The diode responsivity(milliamps/milliwatt) is a fixed known characteristic of the laser diodecalibrated by the manufacturer. The photo diode current is readautomatically through the IEEE-488 bus during the scatter measurementand is used to calculate the laser power.

2. Calibration

FIG. 19 shows a reference sample adapter plate 146 which easily snapsonto the front of the measurement head. This is used to calibratescatter data using a NIST traceable Reference Standard. This factor isreflected in the scaling factor in block 181 of FIG. 18. If the data hasnot been normalized with respect to output power, it is recommended touse a reference measurement taken the same day as the measurement. Thisprocess maximizes accuracy of calibrated measurements, and minimizeserror due to any possible power changes from one measurement to another.To eliminate this possible error the software may be put into anautomatic mode where the sample and reference data are always normalizedwith respect to the output power which is measured and stored with eachdata set.

3. System Profile

The system profile is used to determine how close to specularmeasurements are valid. It also determines the lowest BRDF theinstrument can detect. When the two measurements, system profile versusscatter measurement, are overlaid on a graph, any scatter data thatoverlaps the system profile should be considered to be invalid.

A kinematically mounted reference sample consisting of a highly polishedmirror is provided to allow measurement of the system profile. Thisconfiguration is preferred because of the potential interaction of straylight from the face of the fiber shell and the sample surface. Sincereference samples are available that are significantly better in rmssurface finish than most samples to be measured, this method ofdetermining the system profile is sufficient.

4. Volume of Information

If all the data collected by a 512×512 camera were used, the image anddata file would be over 256KB when stored in binary. The user canspecify what region and resolution of the scatter hemisphere that is tobe collected, analyzed, and stored. This region can range from a singleplane or a two dimensional segment of the scatter hemisphere. A 512×512hemispherical measurement can be displayed under 10 seconds.

F. Data Storage

A resident or remote personal computer is preferred as the optimumcontroller. It allows data acquisition, analysis, and storage to beperformed quickly and easily. In addition, software alterations to matchincreasing demands can be performed with less perturbations than using amicro-controller. Also large volumes of data can be stored on a harddisk or floppy depending their size.

Data can be accumulated in memory and later stored to disk. The amountof data stored in memory will depend on the data acquisition size. At aminimum for in-plane measurements with the minimum BRDF (2° in plane,0.125° resolution), the number of data sets stored in memory will befifty (50).

G. Two Dimensional Plotting

Conventional BRDF plotting software, such as SOFTSCAT-2D™ softwarepackage by BRO is available to plot all two-dimensional data taken by ascatterometer complying with the ASTM standard E1392-90. Also, thesoftware can easily be configured to read other data formats.

The data will be computed and plotted in a variety of ways, including:

β--β_(o) vs BSDF (Harvey Shack)

θ--θ_(o) or θ vs BSDF

θ vs BSDF

polar plots

RMS surface roughness, RMS slope, and power spectral density

linear, log-log, or semi-log format

Some of SOFTSCAT-2D's features include:

ability to "zoom in" on the data

cross-hair positioning

user-definable legends

multiplying and/or adding constants to the data (for comparing differentcurve shapes independent of magnitude)

H. Three Dimensional Plotting

Conventional software, such as, SOFTSCAT-3D, by BRO plots allthree-dimensional data, such as, hemispherical data and surface scanstaken by a scatterometer complying with the ASTM standard E1392-90.Also, the software can easily be configured to read other data formats.

The data can be plotted in a variety of ways, including:

color scaled perspective plots

color maps

contours

cross-sectional plots of the three dimensional data

Other features include:

cubic splines

plotting the log of the data

cross-hair positioning

automatic labeling

I. Graphic Output

Both SOFTSCAT-2D and SOFTSCAT-3D support the following:

over 200 graphics cards

Epson printer output

HPGL plotter output

HPGL file output for input to word processors, such as, Word Perfect

publication quality output

VI. ADDITIONAL MEASUREMENT TECHNIQUES

A. Rapid Surface Scan

Taking data at a few points on a test surface is not always adequate.There are times when an entire surface must be characterized. Very fastsurface scans of a surface can be performed. Since the data is collectedat near video rates, entire surfaces can be scanned very quickly.Instead of signal integration being the limiting factor, now thelimitation is the speed of the x-y scanner.

Note that there are now four dimensions to the BRDF data which includex, y, α, β, where α and β are the hemispherical angles in cosine space.Now the data needs a 4th display parameter which can be supplied bydisplaying the data in rapid succession, much like a movie.

B. Processing and Surface Analysis

A two dimensional grey scale image is produced with the hemisphericalapproach. From the patterns in the image a wide range of imageprocessing and data analysis software can be developed to analyze thenature of the surface and its defects.

A working example of a scatterometer of the present invention has thefollowing features:

                  TABLE 1                                                         ______________________________________                                        MEASUREMENT HEAD                                                              COMPONENT CHARACTERISTICS:                                                    Item         Characteristics                                                  ______________________________________                                        Dimensions   9.5 × 8.5 × 4" without adjustments                   Shutter      Removable cover                                                  Radiation Source                                                                           Laser Diode, 70 mW, 830-840 nm                                                collimated, circulated, wavefront                                             corrected                                                                     computer controlled with temperature &                                        output stabilization                                             Polarization Randomly polarized (Cornu pseudo-                                             depolarizer)                                                     Source Focus Lens                                                                          Adjustable for powered optics                                    & Control                                                                     Fiber Bundle Solid glass fiber bundle                                                      ≈5000 fibers across dia.                                              ≈0.015° resolution/fiber                                       40-60° angular measurement range                          Optical Filter                                                                             Bandpass filter to reduce ambient light                          Beam Dump    Traps specular reflection from                                                measurement surface                                              Camera Head  CID detector array                                                            512 × 512 resolution                                                    15 um × 15 um pixel size                                                11 mm diagonal array                                                          anti-blooming                                                    ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        SCATTEROMETER PARAMETERS                                                      Parameter         Range/Quantity                                              ______________________________________                                        BRDF Range        10.sup.-1 to 10.sup.-5                                      Angular Resolution.sup.1                                                                        0.25° (256 × 256)                                                0.125° (512 × 512)                             Angular Range     2° to ≈60°                            Hemispherical     Forward Scatter 40°-60°                       Measurement       hemispherical circular                                                        section                                                     Wavelength        830°-840°                                     Spot Size         ≈4 mm                                               Incident Angle    7°                                                   ______________________________________                                         .sup.1 Angular resolution is selectable under computer control and is         defined by the spatial resolution digitized from the camera.                  Measurement Time <1 sec                                                       Display Time <2 sec                                                      

                  TABLE 3                                                         ______________________________________                                        DATA ACQUISITION AND SOFTWARE                                                 Parameter   Description                                                       ______________________________________                                        User Interface                                                                           Graphical User Interface (GUI),                                               keyboard, and mouse                                                Measurement                                                                              time <1 second, at least 3 second                                             intervals                                                                     BRDF Range 10.sup.-1 to 10.sup.-5                                             0.125° or 0.25° resolution                                      in-plane BRDF (2° to 40° minimum)                               hemispherical area (out-of-plane)                                  Display    computer screen display time <2                                               seconds                                                                       video display in realtime,                                                    near, mid, and far angle BRDF (peak                                           and/or average)                                                    Analysis   normalization, calibration                                                    TIS, RMS, PSD, AutoCorrelation, BRDF at                                       other wavelengths                                                  Data Storage                                                                             Selectively stores data in the ASTM                                           1392-90& binary formats                                                       Hard disk storage                                                  Graphical Output                                                                         All acquired data can be plotted in 2 or                                      3 dimensions                                                                  Publication quality output                                         ______________________________________                                    

VII. TECHNICAL NOTES AND CALCULATIONS TECHNICAL NOTES Detector Linearity

As mentioned in Section 4.3 the CID Camera will be allowed to saturate.From experiments at BRO the camera can take two orders of magnitude ofpower beyond saturation before the pixels begin to invert. Thisinversion, as described by the manufacturer, is due to the electronicssubtracting out what it believes to be fixed pattern noise. Thisinversion has no effect on the final data since the saturation isdetected well before the inversion, and the data was collected beforethe saturation. Treating the camera in this way is quite common in thelaser profiling market. And completely safe for the camera when thelaser power is kept within a reasonable range, which is wavelengthdependent. The power used for the proposed instrument is orders ofmagnitude within the safety limit.

The laser output power can be controlled to within one (1) percent,through the use of a built in thermoelectric cooler and a feedback powercontrol loop.

Calibration

The two methods of calibration which can be used for normalizing theBSDF data are the Reference Substitution Process method (RSS) and theAttenuated Input Beam Direct Measurement method (ABDM).

These methods are described in detail as follows:

RSS Calibration Method

Currently, BRO scatterometers can automatically calibrate the data bymeasuring a gold calibration standard (which is traceable to a NISTstandard). A gold reference works well with the large spectral rangerequired. The equations are:

BRDF_(S) =[BRDF_(R) *cos(θ_(R))*V_(S) ]/[cos(θ_(i))*cos(Φ_(i))*V_(R) ]

BRDF_(R) =P_(R) /π (Lambertian)

where:

BRDF_(S) =BRDF of the sample

BRDF_(R) =BRDF of the reference standard

P_(R) =reflectivity of the Lambertian reference (known)

θ_(i),Φ_(i) =detector angles in and out of plane relative to the samplenormal

θ_(R),Φ_(R) =detector angles in and out of plane when taking referencemeasurement

V_(i) =voltage from sample scatter at θ_(i) and Φ_(i)

V_(R) =voltage from reference scatter at θ_(R) and Φ_(R) (Φ_(R) =zerodegrees)

The errors in the RSS method are as follows:

Error in the measured reflectivity of the standard itself (constant).

The reference is measured at one detector angle (10 degrees fromspecular) with the assumption that the reference is diffuse and the BRDFcurve is flat over a certain range of angles. In reality, there are noperfectly flat BRDF curves. Even from the most diffuse whites the BRDFcan change ±30% or more.

The contribution of these errors is constant; this means that they alterthe level of the BSDF curve and not the shape of the curve. An advantageto this method is that some errors, or other factors in the dataacquisition process common to taking data from the standard and the testsample, are normalized out. Past data taken on three different BROscatterometers at the customer site with the same sample (Martin Black)show this calibration technique gives consistent results whenimplemented correctly.

ABDM Calibration Method

The Absolute method uses a reference mirror to direct the incident beamdirectly into the detector, or to catch the beam with no sample orreference mirror in place. The equations are:

BRDF_(S) V_(S) A_(S) R² /[V_(T) A_(T) *cos(θ_(i))*cos(Φ_(i))*a*T]where

θ_(i) Φ_(i) =detector angles in and out of plane relative to the samplenormal

Vs=voltage output when measuring the test sample

V_(T) =voltage output when all radiation incident on sample enters thesample detector when measuring total power

A_(T) =attenuation required to allow all power on detector (positiveexponent)

a=aperture area

A_(S) =attenuation at θ_(S), Φ_(S)

R=distance from collecting optics to sample

T=reflectivity of the reference mirror (T=1.0, for the straight throughmethod).

The errors in the ABDM method are as follows:

error in normalizing out attenuation which is wavelength dependent(constant)

error in measuring "R" and "a" which make up the solid angle

error in determining that all the signal from the source is collected bythe detector, and no significant additional stray light is present.

linearity of the sample and reference detectors, even when theattenuators are in place

For the laser sources, the RSS calibration method cited above will beused.

Stray Light Control

There are two sources of stray radiation in any scatterometer: theinstrument profile, and the illumination beam. The instrument profile isa fixed noise produced by the interaction between the forward scatterfrom the illumination optics (chiefly the last optic before the sample)and the collection system's field-of-view. Its primary effect is tolimit one's ability to make near-specular measurements. However, this isonly a limitation when testing specular samples. When testing diffusesamples, the instrument profile is generally not a problem.

There are two ways to reduce the instrument profile: decrease the sourcesize or beam diameter, or decrease the collecting optic's field-of-view.Adjusting the source size or beam diameter changes the illuminated spotsize on the primary mirror. A smaller collector FOV reduces the angle atwhich that spot is seen by the detector. The BRO scatterometers optimizethese parameters in order to achieve an optimum spot size and a smallinstrument profile.

The other source of stray light is the reflected beam. This arises whenthe beam strikes walls or support structures. If there are no walls,this source of stray light from internal sources is reduced. To avoidstray light from the support structure, it is best to underfill thesample. However, this is not always possible. In the "over-illumination"case, the beam spills off the sample onto the surrounding supportstructure. To minimize stray radiation from the support structure, aspecial sample holder has been designed. By eliminating most of theholder structure around the sample, controlling the overilluminationbeam size (focus adjust), and using beam dumps, BRO has been able tominimize all the instrument sources of stray light.

CALCULATIONS

The CID camera chosen has a full well capacity of 450,000 electrons,which is the saturation point. Thus the objective is when the BRDF is atlowest required to measure (10⁻⁵) it is desired that a pixel elementaccumulate on the order of 250,000 to 450,000 electrons. For a safetyfactor a goal of 450,000 electrons is assumed.

At the minimum BRDF_(min) the signal in electrons on the camera will be,##EQU1## where: t=frame (integration time) [sec]

φ_(i) =input laser power [watts]

BRDF_(min) =10⁻⁵ [steradians⁻¹ ]

Ω=solid angle [steradians]

γ=throughput ratio of the fiber

R=detector responsivity [amps/watts]

q=charge of electron 1.6 E-19 [coulombs]

N=number of collected electrons

From among commercially available laser diodes two choices are made, atdifferent locations in the available spectrum. Substitutingrepresentative values for these wavelengths:

t=0.200 sec

BRDF=1×10^('15)

Ω=(0.25/57.3)²

γ=0.4

q=1.6E-19

φ_(i) (840)=70 mw

φ_(i) (670)=3 mw

R (840)=0.065 amps/watts

R (670)=0.115 amps/watts

1 amp=0.6 E¹⁹ electron/sec

From the CID responsivity curve shown below, it is seen that the camerasensitivity peaks around 600 nm, while commercialized diodes have morepower at higher wavelengths.

Thus, from the above the product φ_(i) ·R must be maximized. ##EQU2##

From the CID responsivity curve shown the responsivities at thesewavelengths are:

R=f(λ nm)

R(670)=0.115 Amps/Watt

R(840)=0.065 Amps/Watt

using the above equation and values we get

N(670)=32844 electrons

N(840)=433160 electrons

Previously it was assumed that 4.5×10⁵ electrons were desired. Thus, the670 nm diode is not sufficient but, the 840 nm 70 mW is enough. Thusdiode power can be controlled from a few micro-watts to 70 mW bycontrolling the diode on-off time (down to 10 ns, through the use of thediode controller).

The foregoing description of a preferred embodiment and best mode of theinvention known to applicant at the time of filing the application hasbeen presented for the purposes of illustration and description. It isnot intended to be exhaustive or to limit the invention to the preciseform disclosed, and obviously many modifications and variations arepossible in the light of the above teaching. The embodiment was chosenand described in order to best explain the principles of the inventionand its practical application to thereby enable others skilled in theart to best utilize the invention in various embodiments and withvarious modifications as are suited to the particular use contemplated.It is intended that the scope of the invention be defined by the claimsappended hereto.

I claim:
 1. An apparatus for collecting light from a common light sourcewhich comprises:a plurality of individual fiber optic elements one endof each of which is fixed in spaced relation to each other and an otherend of each of which is arranged in an indexed array, the plurality offiber optic elements have a common field of view and a face at the oneend of each of which is fixed in a curved surface having a radius ofcurvature such that the longitudinal axis of each fiber optic element atthe one end substantially converges at the common light source which,with respect to each fiber optic element, radiates light normal to aplane which is both tangent to the face of the fiber optic element atthe normal axis and perpendicular to the longitudinal axis of the fiberoptic element, whereby the light radiating angularly from the lightsource is received simultaneously at the one end by each fiber opticelement and is transmitted to the other end.
 2. The apparatus as recitedin claim 1, wherein the curved surface is a full hemisphere.
 3. Theapparatus, as recited in claim 2, further comprising:a rigid hemi-domewhich has a plurality of openings in each one of which is secured oneend of one of the fiber optic elements.
 4. The apparatus, as recited inclaim 2, wherein the plurality of fiber optic elements form a taperedbundle.
 5. The apparatus, as recited in claim 4, wherein the taperedbundle is fused.
 6. The apparatus, as recited in claim 1, wherein thecurved surface is a spherical segment.
 7. The apparatus, as recited inclaim 6, whereinthe plurality of fiber optic elements is a tapered fiberoptic bundle, and the spherical surface is in the tapered portion of thebundle.
 8. The apparatus, as recited in claim 7, wherein the curvedsurface is in the one end which is a wide end of the bundle and theother end is a narrow end of the bundle.
 9. The apparatus, as recited inclaim 7, wherein the tapered fiber optic bundle is a unitary bundlewhich has a wide central portion, a first tapered portion in which thespherical surface is located at the one end and a second tapered portionwhich terminates at the other end, the first and second tapered portionsbeing tapered in opposite directions.
 10. The apparatus, as recited inclaim 9, wherein the bundle is fused.
 11. The apparatus, as recited inclaim 9, wherein the first and second tapered portions each terminate atwide ends which are bonded together to form the wide central portion.12. The apparatus, as recited in claim 11, wherein the first and secondtapered portions are rotated 45° relative to each other prior to beingbonded.
 13. The apparatus, as recited in claim 9, wherein the unitarybundle is a double tapered fused fiber optic bundle.
 14. The apparatus,as recited in claim 13, wherein the light source is a point on a surfaceto be tested and further comprising:a path cut through the bundle fortransmitting therethrough light from a remote light source to illuminatethe point.
 15. The apparatus, as recited in claim 14, furthercomprising:means to suppress a specular beam reflected from the point.16. The apparatus as recited in claim 7, further comprising:the bundlehaving an upper portion, a longitudinal slot in the upper portion of thebundle for input of a source beam of various angles of incidence. 17.The apparatus, as recited in claim 1, wherein the curved surface is asegment fixing the one ends of all the fiber optic elements in a commonplane containing their longitudinal axis.
 18. The apparatus, as recitedin claim 17, further comprising:an aperture block which has a pluralityof radial spaced passages in each one of which is secured one fiberoptic element.
 19. The apparatus, as recited in claim 1, furthercomprising:the fiber optic elements are a bundle and a narrow bandcoating is on the outside of the fiber bundle.
 20. The apparatus ofclaim 1 wherein the other end is flat.
 21. A method of manufacturing afiber optic bundle comprising the steps of:a) forming a tapered fiberoptic bundle, and b) cutting a concave face in the tapered portion ofthe bundle such that for each fiber in the bundle a plane tangent to theface at the normal axis to the fiber is also perpendicular to thelongitudinal axis of the fiber.